Using the CW Doppler method in an ultrasonic diagnostic system internally exposes a subject to a continuous ultrasonic wave, and determines the speed of an object in motion, such as the blood flow, by using the Doppler effect of ultrasonic echo signals. The B-mode imaging method applies ultrasonic pulses to a subject and displays a tomographic image of the subject according to the intensity of a ultrasonic echo. The B-mode imaging method is often applied together with the CW Doppler method for convenience because B-mode imaging provides an operator with tomographic images of the subject. The CW Doppler and B-mode imaging methods are conventionally applied using separate probes. It would be much more convenient if a probe, such as the array probe used for B-mode imaging, could be commonly applied to both methods. However, such application poses the following problems.
The B-mode imaging method uses a transmission waveform having a narrow pulse width to improve resolution. Note not decreased transmission power means decreased echo reception power, which adversely affects the signal-to-noise ratio and picture quality. To cope with this, the transmission pulse amplitude must be increased to increase the transmission power. To increase the transmission pulse amplitude, the output voltage of the transmitting driver must be raised (to 80 V or higher, for example). Conversely, the CW Doppler method utilizes a continuous wave for signal transmission. Therefore, the output voltage for the transmitting driver must also be lowered (to several volts, for example) for two reasons: a safe limits for living bodies, and operating power limits for the probe.
When applying the CW Doppler and B-mode imaging methods together using a commonly applicable array probe, tomographic images of the subject are required for observation while measuring the blood flow by using the CW Doppler method. Temporarily switching to the B-mode imaging method and returning to the CW Doppler method immediately thereafter to resume blood flow measurement is also required. To enable this, the output voltage of the transmitting driver must be instantaneously switched to high voltage, low voltage, and back to the high voltage. For the configuration needed to enable such changeover, a transmitting driver to which high voltage is supplied must be considered, and by controlling the output amplitude of its final stage amplifier to output low and high voltages. In this case, if small amplitude output must be provided in the CW mode from a high-voltage power supply, a problem is posed by the excessively high temperatures generated in the final stage amplifier. Consequently, to solve this problem, switching the supply voltage must be considered so that the supply voltage to the transmitting driver is lowered in correspondence to low-voltage output or is raised for high-voltage output. However, this switching method requires the charging and discharging of bypass capacitors positioned in the circuits, which make instantaneous switching difficult. It takes at least tens to several hundreds of miliseconds for effective switching. This means that such a transitional pause must be placed before and after B-mode imaging, making it difficult to apply the B-mode imaging method without practically interrupting the application of the CW Doppler method.